1. Field of the Invention
This invention relates to an exchange coupling film utilizing the exchange coupling between an antiferromagnetic film and a ferromagnetic film and magnetoresistive elements such as magnetic field sensors and reproducing magnetic heads which are provided with such an exchange coupling film as mentioned above.
2. Description of the Related Art
The study of a magnetic head using a magnetoresistive element as a reproducing head for the sake of high-density magnetic recording has been heretofore studied. At present, a thin film of 80 at %Ni-20 at %Fe alloy (otherwise known as xe2x80x9cPermalloyxe2x80x9d). is generally used as a magnetoresistive material. As alternatives to this material, artificial lattice films such as of (Co/Cu)n which exhibits a giant magnetoresistive effect and spin valve films have been attracting attention in recent years.
Incidentally, since magnetoresistive films using these materials have magnetic domains, the Barkhausen noise originating by multi-domain activities poses a serious problem in the way to the commercialization the films. Various methods for imparting a single domain to the magnetoresistive films have been being studied from various angles. A method which resides in controlling in a specific direction the domains of a magnetoresistive film by utilizing the exchange coupling between a magnetoresistive film which is a ferromagnetic film and an antiferromagnetic film is counted among these methods. As the antiferromagnetic material used for exchange coupling, a xcex3-FeMn alloy has been heretofore finding-wide recognition (disclosed in U.S. Pat. No. 4,103,315 and U.S. Pat. No. 5,315,468, for example).
Further, the technique utilizing an exchange coupling between an antiferromagnetic film and a ferromagnetic film for pinning a magnetization of a magnetic film of a spin valve film has been disseminating in recent years. Also for this purpose, a xcex3-FeMn alloy has been used as an antiferromagnetic material.
The xcex3-FeMn alloy, however, is deficient in resistance to corrosion, particularly to the corrosion due to water, and is at a disadvantage in suffering the yield of elements being produced to be notably lowered and the force of exchange coupling with a magnetoresistive element to be deteriorated with time as by the corrosion during the course of fabrication of the magnetoresistive element and by the corrosion by the moisture in the air.
The antiferromagnetic film which is made of the xcex3-FeMn alloy is problematic in respect that the force of exchange coupling with the ferromagnetic film is markedly altered by the temperature environment. Since the temperature of element parts of the magnetic head ultimately rises to the neighborhood of 80xc2x0 C. during the operation of the magnetic head, the blocking temperature at which the. force of exchange coupling between the ferromagnetic film and the antiferromagnetic film is wholly lost ought to be as high as permissible. Since the blocking temperature of the xcex3-FeMn alloy system is not higher than 200xc2x0 C., the antiferromagnetic film under discussion is at a disadvantage in exhibiting only inferior lasting reliability.
U.S. Pat. No. 4,103,315, for example, discloses cases of using antiferromagnetic films made of such xcex3-Mn alloys as PtMn and RhMn other than the xcex3-FeMn alloy and also cases of using antiferromagnetic films made of such oxides as NiO. The anti-ferromagnetic films made of such xcex3-Mn alloys as PtMn and RhMn, however, exhibit no sufficient force of exchange coupling with a ferromagnetic film. In contrast, the antiferromagnetic films made of such oxides as NiO exhibit inferior thermal stability and exhibit unstable force of exchange coupling with a ferromagnetic film at such high temperatures as exceed about 100xc2x0 C. Further, such oxide systems as NiO exhibit high electric resistance, permit no direct derivation of an electrode therefrom, and entail the disadvantage of complicating the construction of an element.
U.S. Pat. No. 5,315,468 discloses an observation that when an antiferromagnetic film is formed of such a xcex8-Mn alloy as NiMn which has a face-centered tetragonal (fct) crystal structure, the force of exchange coupling between the antiferromagnetic film and a ferromagnetic film is not degraded even in a range of high temperatures.
The antiferromagnetic film of this nature, as deposited, exhibits a very small force of exchange coupling with a ferromagnetic film and, for the sake of acquiring a fully satisfactory force of coupling, must undergo a heat treatment at a high temperature in the neighborhood of 250xc2x0 C. by all means. When the antiferromagnetic film of this sort is adopted, therefore, the process of production is complicated to the extent of lowering the yield of production and degrading the reliability of the product.
Antiferromagnetic films have been heretofore used, as described above, for attaining exchange coupling with ferromagnetic films as by abating the Barkhausen noise inherent in magnetoresistive elements. The conventional antiferromagnetic films, however, suffer from deficiency in force of exchange coupling with a ferromagnetic film or in resistance to corrosion particularly at elevated temperatures and incur difficulty in producing exchange coupling films of fully satisfactory reliability with a high yield.
The purpose of this invention is to clear the problems encountered as described above.
An object is the provision of an exchange coupling film which is provided with an antiferromagnetic film exhibiting ample force of exchange coupling with a ferromagnetic film even in a range of high temperature and, at the same time, excelling in resistance to corrosion, produced by a simple process, and possessed of good reliability and a magnetoresistive element which is provided with the exchange coupling film and enabled to produce stable output for a long time.
A first exchange coupling film according to this invention is an exchange coupling film obtained by laminating an antiferromagnetic film and a ferromagnetic film and characterized in that at least a portion of the antiferromagnetic film has a face-centered cubic crystal structure and made of an IrMn alloy of a composition represented by the general formula, IrxMn100xe2x88x92x, wherein x stands for a value by at % satisfying the expression, 2xe2x89xa6xxe2x89xa680.
The second exchange coupling film according to this invention is an exchange coupling film obtained by laminating an antiferromagnetic film and a ferromagnetic film and characterized in that the antiferromagnetic film is made of an IrMn alloy of a composition represented by the general formula, IrxMn100xe2x88x92x, wherein x stands for a value by at % satisfying either of the expressions, 2xe2x89xa6xxe2x89xa635 and 60xe2x89xa6xxe2x89xa680.
The third exchange coupling film according to this invention is an exchange coupling film obtained by laminating an antiferromagnetic film and a ferromagnetic film and characterized in that the antiferromagnetic film is made of an IrMn alloy of a composition represented by the general formula, (IrxMn1xe2x88x92x,)100xe2x88x92yFey, wherein xxe2x80x2 stands for a value by atomic ratio satisfying the expression, 0.02xe2x89xa6xxe2x80x2xe2x89xa60.80, and y for a value by at % satisfying the expression, 0xe2x89xa6yxe2x89xa630.
Then, the magnetoresistive element of this invention is characterized by being provided with such an exchange coupling film as mentioned above and an electrode for feeding an electric current at least to the ferromagnetic film in the exchange coupling film.
To be specific, this invention is characterized by using an IrMn alloy possessed of specific crystal structure and composition for the antiferromagnetic film in the exchange coupling film.
Now, this invention will be described in detail below. The first exchange coupling film according to this invention is provided with a basic structure formed by laminating an antiferromagnetic film made of an IrMn alloy and a ferromagnetic film.
This first exchange coupling film is enabled to acquire an amply large force of exchange coupling even in a range of high temperatures particularly by laminating an antiferromagnetic film at least a portion of which has a face-centered cubic crystal structure and made of an IrMn alloy of a composition represented by the general formula, IrxMn100xe2x88x92x, (2xe2x89xa6xxe2x89xa680) and a ferromagnetic film.
The IrMn alloy which is possessed of the face-centered cubic crystal structure exhibits a high Nxc3xa9el temperature. When it is utilized in the exchange coupling film of such a basic structure as mentioned above, it manifests a high blocking temperature therein and consequently imparts improved reliability to the ultimately obtained exchange coupling film and manifests a highly satisfactory force of exchange coupling with the ferromagnetic film.
Further, particularly when the exchange coupling between a ferromagnetic film and an antiferromagnetic film is utilized for controlling magnetic domains of the ferromagnetic film as a magnetoresistive film and for magnetically pinning a pinned layer of a spin valve film and the like, the IrMn alloy which has a face-centered cubic crystal structure proves advantageous in terms of the lattice matching property to be exhibited to a ferromagnetic film which has a face-centered cubic crystal structure or a hexagonal closest packing crystal structure.
Conversely, in the IrMn alloy which has a face-centered tetragonal crystal structure, the ratio of the lattice constants of the c axis and the a axis, c/a, is 1.355, a very large magnitude, and moreover the lattice constant in the direction of the a axis is less than about 0.3 nm. This IrMn alloy, therefore, exhibits a poor lattice matching property to a ferromagnetic film which generally forms a face-centered cubic crystal structure having a lattice constant of about 0.35 nm and does not easily produce an ample force of exchange coupling therewith.
In the first exchange coupling film according to this invention, the IrMn alloy which has a face-centered cubic crystal structure as described above and a composition represented by the general formula, IrxMn100xe2x88x92x, (2xe2x89xa6xxe2x89xa680), is used for the antiferromagnetic film. The reason for this specific composition is that the corrosion resistance of the IrMn alloy tends to decline when the Ir content of this alloy is small and the antiferromagnetism tends to be weak when the Ir content is large. For the sake of this invention, the Ir content of the IrMn alloy is preferably in this range, 5xe2x89xa6xxe2x89xa640.
The second exchange coupling film according to this invention is an exchange coupling film obtained by laminating an antiferromagnetic film and a ferromagnetic film and characterized in that the antiferromagnetic film is made of an IrMn alloy having a composition represented by the general formula, IrxMn100xe2x88x92x, and an Ir content x in either of the ranges, 2 to 35 at % and 60 to 80 at %.
The IrMn alloy generally has the face-centered tetragonal crystal structure thereof stabilized when the Ir content, x, is in the range of 35xe2x89xa6xxe2x89xa660.
In the second exchange coupling film according to the present invention, it is more advantageous to use the IrMn alloy having a composition represented by the general formula, IrxMn100xe2x88x92x (2xe2x89xa6xxe2x89xa635, 60xe2x89xa6xxe2x89xa680) or better still IrxMn100xe2x88x92x (5xe2x89xa6xxe2x89xa635).
When an IrMn alloy is epitaxially grown on a film such as of Cu having a face-centered cubic crystal structure or on a magnetoresistive film formed mainly of Fe, Co, or Ni or an alloy thereof, however, an antiferromagnetic film which is formed of an IrMn alloy having a face-centered cubic crystal structure and a composition in which the Ir content is in the range, 35xe2x89xa6xxe2x89xa660, can be produced.
In this invention, so long as the antiferromagnetic film is formed of an IrMn alloy having a face-centered cubic crystal structure, this film incurs no particular hindrance even when the IrMn alloy has a composition the Ir content of which is in the range, 35xe2x89xa6xxe2x89xa660, as in the first exchange coupling film according to this invention.
The antiferromagnetic film in the third exchange coupling film according to this invention has a composition represented by the general formula, (Irx, Mn1xe2x88x92x,)100xe2x88x92yFey, wherein xxe2x80x2 stands for a value by atomic ratio satisfying the expression, 0.02xe2x89xa6xxe2x80x2xe2x89xa60.80, and y for a value by at % satisfying the expression, 0xe2x89xa6yxe2x89xa630.
The antiferromagnetic film in the third exchange coupling film according to this invention is formed of an alloy composition having Fe added to the IrMn alloy forming the antiferromagnetic film in the first exchange coupling film according to this invention.
The reason for defining the value of xxe2x80x2 to be not less than 0.02 is that the corrosion resistance of the antiferromagnetic film is unduly low when the Ir content is less than 0.02 and the blocking temperature of the antiferromagnetic film is unduly low when the Ir content exceeds 0.8. More preferably, the value of xxe2x80x2 is in the range, 0.05xe2x89xa6xxe2x80x2xe2x89xa60.40.
Fe fulfills the role of enhancing the lattice matching property of the antiferromagnetic film with the ferromagnetic film and exalting the force of exchange coupling. The value of y must be less than 30 because the corrosion resistance is notably lowered when the value of y exceeds 30. More preferably, the value of y is in the range, 0.01xe2x89xa6yxe2x89xa625.
Likewise in the third exchange coupling film according to this invention, both the antiferromagnetic film and the ferromagnetic film which form the exchange coupling film preferably form a face-centered cubic (fcc) crystal structure.
Further, in an embodiment of the present invention, since both the ferromagnetic film and the antiferromagnetic film form a (111) plane orientation, the ferromagnetic film having a hexagonal crystal structure may be used. Incidentally, as stated above with respect to the first and the second exchange coupling film according to this invention, the IrxMn1xe2x88x92x, type alloy in bulk has a face-centered tetragonal (fct) crystal structure when the composition of the alloy has an Ir content, xxe2x80x2, in the range, 0.35xe2x89xa6xxe2x80x2xe2x89xa60.60. This IrMn type alloy of the face-centered tetragonal crystal structure has such a small lattice constant, a, of about 0.273 nm and such a fairly large c/a ratio of 1.355. In contrast, the ferromagnetic film having a face-centered cubic (fcc) crystal structure has a lattice constant of about 0.35 nm. When the (Irx, Mn1xe2x88x92x,)100xe2x88x92yFey (0.35xe2x89xa6xxe2x80x2xe2x89xa60.60) type alloy is used as the antiferromagnetic film, therefore, it is predicted that this film will exhibit a poor lattice matching property to the ferromagnetic film and will not easily produce a satisfactory force of exchange coupling.
When the (Irx, Mn1xe2x88x92x,)100xe2x88x92yFey type alloy having a composition whose Ir content, xxe2x80x2, is in the range, 0.35xe2x89xa6xxe2x80x2xe2x89xa60.60, however, is epitaxially grown on a film such as of Cu having a face-centered cubic crystal structure or on a magnetoresistive film formed mainly of Fe, Co, or Ni or an alloy thereof, there can be formed an antiferromagnetic film which has a face-centered cubic crystal structure.
In the third exchange coupling film, the distribution of Fe concentration along the direction of film thickness of the antiferromagnetic film formed of an (Irx, Mn1xe2x88x92x,)100xe2x88x92yFey type alloy may be uniform or ununiform (composition-varied film). For example, the Fe concentration may be higher on the surface of the antiferromagnetic film or the interface with the ferromagnetic film or in the central part in the direction of thickness of the antiferromagnetic film.
Preferably , from the viewpoint of force of exchange coupling and corrosion resistance, however, the Fe concentration is high in the neighborhood of the interface between the antiferromagnetic film and the ferromagnetic film. As respects the manner of variation of the Fe concentration in the antiferromagnetic film, the variation may be continuous or stepwise.
Further, in the first, the second, and the third exchange coupling film according to this invention, the IrMn alloy which is used in the antiferromagnetic film may incorporate therein such additive components as Ni, Cu, Ta, Hf, Pd, Ti, Nb, Cr, Si, Al, W, Zr, Ga, Be, In, Sn, V, Mo, Re, Co, Ru, Rh, Pt, Ge, Os, Ag, Cd, Zn, Au, and N.
The antiferromagnetic film contemplated by this invention already acquires highly satisfactory corrosion resistance owing to the use of an IrMn alloy having such crystal structure and composition as described above, it is allowed to have the corrosion resistance improved further by the incorporation of such additive components.
If these additive components are incorporated in an unduly large amount, however, they will possibly degrade the force of exchange coupling of the exchange coupling film. Appropriately, therefore, the amount of the additive components to be incorporated is defined to be not more than 50 at % based on the composition represented by the general formula, IrxMn100xe2x88x92x (2xe2x89xa6xxe2x89xa680). In the case of Cu, Ta, Hf, Ti, Nb, Cr, Si, Al, W, Zr, and Mo, this amount is properly not more than 30 at %. In the case of N, the amount is not more than 20 at %.
The same remarks hold good for the IrMnFe alloy in the third exchange coupling film. If the amount of the additive components to be incorporated exceeds 50 at %, the force of exchange coupling of the exchange coupling film will be unduly low.
Further in the exchange coupling film of this invention, the antiferromagnetic film which is formed of the IrMn alloy (inclusive of the IrMnFe alloy) advantageously is possessed at least partly of an ordered phase. This is because the Nxc3xa9el point is elevated by ordering the atomic arrangement of the antiferromagnetic film of the IrMn alloy, consequently the blocking temperature of the exchange coupling film is heightened and the reliability of this film is exalted and, at the same time, the force of exchange coupling between the antiferrbmagnetic film and the ferromagnetic film is augmented.
When the antiferromagnetic film is formed of an IrMn alloy having a face-centered cubic crystal structure as in the present invention, generally the antiferromagnetic film as deposited state predominantly has an unordered phase. By a heat treatment performed at a temperature in the approximate range of 100 to 300xc2x0 C., however, the antiferromagnetic film is enabled to form an ordred phase, concretely an a Cu3Au type ordered phase. Here, the formation of this. ordered phase can be easily confirmed by the X-ray diffraction analysis of the heat-treated film.
This invention does not particularly discriminate the ferromagnetic film on any account. In terms of the lattice matching property thereof with the antiferromagnetic film, however, it is properly a magnetoresistive film is preferable to form a face-centered cubic crystal structure or a hexagonal closest packing crystal structure. The magnetoresistive films for this description include anisotropic magnetoresistive films and giant magnetoresistive films such as artificial lattice films, spin valve films, and granular films, for example. Such magnetoresistive films as are mainly formed of an alloy containing at least one metal selected from among Fe, Co, and Ni are concrete examples. The ferromagnetic film is preferable to contain Fe. The Fe thus contained in the ferromagnetic film brings about the advantage of augmenting the force of exchange. coupling between the ferromagnetic film and the antiferromagnetic film.
The magnetoresistive film which particularly has Co or a Co alloy as a main component thereof, when laminated on an antiferromagnetic film formed of an IrMn alloy having a face-centered cubic crystal structure, can form an exchange coupling film which has an enough high blocking temperature for a magnetic head application.
When an artificial lattice film or a spin valve film which has a multilayer structure including the combination such as of a Co type alloy magnetic film and a Cu nonmagnetic film is used as a magnetoresistive film, this magnetoresistive film is utilized highly advantageously for a magnetic head, for example, because it acquires a large rate of change in resistance and moreover exhibits good thermal stability.
Now, the thermal stability of the magnetoresistive film which is possessed of such a multilayer structure as mentioned above will be described further.
In a multilayer structure comprising a NiFe magnetic film and a Cu nonmagnetic film, for example, Ni and Cu form a solid solution in all composition range. When this multilayer structure is exposed to a temperature of about 200xc2x0 C. as during the fabrication process of a magnetoresistive element, therefore, diffusion occurs between the NiFe magnetic film and the Cu nonmagnetic film and the rate of change in resistance of the magnetoresistive film is lowered inevitably. In contrast, in a multilayer structure comprising a Co type alloy magnetic film and a Cu nonmagnetic film, Co and Cu hardly form a solid solution region. Even when the magnetoresistive film is heated to about 350xc2x0 C. during the course of fabrication of a magnetoresistive element, therefore, the rate of change in resistance of this film shows substantially no decline.
Optionally, this invention allows a ferromagnetic film made of Co or a Co type alloy to be inserted in the interface between a ferromagnetic film containing no Co and an antiferromagnetic film to heighten the blocking temperature of an exchange coupling film to be produced.
In this case, the ferromagnetic film containing no Co and a ferromagnetic film made of Co or a Co type alloy may be superposed severally on the opposite surfaces of an antiferromagnetic film.
Further, this invention allows the ferromagnetic film to incorporate therein additive components with a view to improving the magnetic properties of the ferromagnetic film and augmenting the lattice matching property of this film with an antiferromagnetic film made of an IrMn alloy.
From the same point of view, the ferromagnetic film made of a NiFe type alloy is allowed to incorporate therein additive components. In this case, the incorporated additive components need not be distributed throughout the whole volume of the ferromagnetic film but may properly be distributed at least in the proximity of the interface for the sake of augmenting the lattice matching property thereof with the antiferromagnetic film.
The thickness of the antiferromagnetic film in the present invention has no particular restriction except the requirement that it permit the film to manifest necessary antiferromagnetism. For the purpose of enabling the antiferromagnetic film to acquire large force of exchange coupling, the thickness of the antiferromagnetic film ought to be larger than that of the ferromagnetic film. From the viewpoint of ensuring the stability of the force of exchange coupling after the heat treatment, this thickness is appropriately not more than about 15 nm, or preferably not more than about 10 nm. Further, from the same view point the thickness of the ferromagnetic film is preferably less than about 3 nm. These antiferromagnetic film and ferromagnetic film are only required to form the exchange coupling by being at least partly laminated on each other.
Further, the thickness of the exchange coupling film of the invention is preferably more than about 3 nm, and the thickness of the ferromagnetic film to be pinned is preferably more than about 1 nm.
The exchange coupling film of this invention is formed on a substrate, for example, by any of the known film-forming methods such as, for example, the vacuum deposition method, sputtering method, and MBE method. In this case, for the purpose of imparting unidirectional anisotropy to the exchange coupling between the antiferromagnetic film and the ferromagnetic film, the exchange coupling film may be formed in a magnetic field or it may be heat-treated in a magnetic field. The heat treatment given herein is further effective in forming the ordered phase mentioned above.
When the ferromagnetic film contains Fe in this case, the Fe during the course of the heat treatment is diffused from the ferromagnetic film toward the antiferromagnetic film to form a diffused layer of Fe along the interface between the two films, with the result that the Fe concentration will be increased along the interface and the force of exchange coupling further augmented.
Incidentally, such diffusion of Fe from the ferromagnetic film toward the antiferromagnetic film as mentioned above may be utilized for forming an IrMnFe antiferromagnetic film of the third exchange coupling film according to this invention by laminating a ferromagnetic film and an IrMn film containing no Fe and then annealing the resultant laminated films. When a ferromagnetic film containing no Fe is used, an IrMnFe antiferromagnetic film may be formed by interposing a layer containing Fe as a main component between the ferromagnetic film containing no Fe and an IrMn antiferromagnetic film containing no Fe. The layer containing Fe as a main component and inserted in the interface as mentioned above properly has a thickness of not more than about 5 nm, preferably not more than about 2 nm. When a Fe layer having a thickness of not less than monoatomic layer is present in the interface between an IrMn film containing no Fe and a ferromagnetic film containing no Fe, it contributes to augment the force of exchange coupling.
By the same token, the insertion of such a Fe layer as mentioned above in the interface enables an IrMnFe layer to acquire an augmented force of exchange coupling even when the ferromagnetic film either contains no Fe or contains Fe only in a small amount. The Fe layer in this case manifests a necessary effect so long as it has a thickness of not less than monoatomic layer. If the Fe layer has a thickness exceeding about 5 nm, the force of exchange coupling will be unduly low.
A material of the substrate of the exchange coupling film of the present invention does not be particularly restricted. An amorphous substrate such as a glass or resin, a single crystal such as Si, MgO, Al2O3 or various ferrites, an orientation substrate and a sintered substrate may be used. Further, to improve the cristallinity of the antiferromagnetic film or the ferromagnetic film, an underlayer having a thickness of about 1 to 100 xcexcm may be provided on the substrate. A material for forming the underlayer has no particular restriction except for the requirement that it is capable of improving the crystallinity of the antiferromagnetic film or the ferromagnetic film. A noble metal such as Pd or Pt, an amorphous metal such as CoZrNb, and a metal and alloy thereof having a face-centered cubic crystal structure may be used.
The magnetoresistive element of this invention comprises such an exchange coupling film as described above and an electrode adapted to feed an electric current to at least a ferromagnetic film of the exchange coupling film. As the raw material for this electrode, Cu, Ag, Al, and alloys thereof are available. The electrode may be formed in such a manner as to contact a ferromagnetic film either directly or through the medium such as of an antiferromagnetic film.
The exchange coupling film of this invention is provided with an exchange coupling film which is capable of producing such a large force of exchange coupling as mentioned above. Thus, it can be utilized for various devices such as magnetic field sensors and regenerating magnetic heads which make use of a magnetoresistive element.
In the magnetoresistive element of this invention, the force of exchange coupling between an antiferromagnetic film and a ferromagnetic film can be utilized not only for the control of magnetic domains of a magnetoresistive film which is a ferromagnetic film, namely the removal of Barkhausen noise from a magnetoresistive element, but also for allowing artificial lattice films and spin valve films as magnetoresistive film to be magnetically fastened.